The action may be similar to the Handy Gun and the 088 and 099 shotguns in which the trigger guard is retained by the trigger pivot pin. The pin then has to be drifted out to drop the trigger guard down out of the frame. I don't think I've ever had a 1900 apart myself, so I can't verify that it's the same. A picture of the whole side of the action would verify or disprove this idea.